Add CREATE TABLE AS support (#206)

We parse it as a regular `CREATE TABLE` statement
followed by an `AS <query>`, which is how BigQuery works:
https://cloud.google.com/bigquery/docs/reference/standard-sql/data-definition-language#create_table_statement


ANSI SQL and PostgreSQL only support a plain list of columns
after the table name in a CTAS
    `CREATE TABLE t (a) AS SELECT a FROM foo`

We currently only allow specifying a full schema with data
types, or omitting it altogether.

https://www.postgresql.org/docs/12/sql-createtableas.html
https://jakewheat.github.io/sql-overview/sql-2016-foundation-grammar.html#as-subquery-clause


Finally, when no schema is specified, we print empty parens after a
plain `CREATE TABLE t ();` as required by PostgreSQL, but skip them
in a CTAS: `CREATE TABLE t AS ...`. This affects serialization only,
the parser allows omitting the schema in a regular `CREATE TABLE` too
since the first release of the parser:
7d27abdfb4/src/sqlparser.rs (L325-L332)

Co-authored-by: Nickolay Ponomarev <asqueella@gmail.com>

tests/sqlparser_mysql.rs

@@ -77,7 +77,7 @@ fn parse_show_columns() {
         Statement::ShowColumns {
             extended: false,
             full: false,
-            table_name: table_name,
+            table_name,
             filter: Some(ShowStatementFilter::Where(
                 mysql_and_generic().verified_expr("1 = 2")
             )),